1. Field of The Invention
This invention relates to a quaternary ammonium salt primer for bonding of surfaces with cyanoacrylate adhesives, and to a method of bonding surfaces utilizing such primer, particularly surfaces which are characterized by low surface energy values, such as polyolefins, or which are otherwise difficult to bond with conventional cyanoacrylate adhesives.
2. Description of the Related Art
In the adhesives field, cyanoacrylate adhesive compositions have come into wide-spread usage, due to their rapid fixturing properties and their high bonding strength characteristics, e.g., tensile shear strength. Such compositions typically fixture within a few minutes or less, and exhibit tensile shear strength values which for metal-to-metal bonding may be on the order of from about 1,000 to 3,000 psi, as measured by the lap shear strength determination method of ASTM D-1002-64.
While cyanoacrylate adhesives have enjoyed usage for bonding of a wide variety of substrate materials, such as metals, wood, glass, ceramics, metals, semi-metals, and combinations and composites thereof, there exist a number of substrate materials of construction which are extremely difficult to bond with cyanoacrylate adhesives or, indeed, with most other adhesives. Such difficult-to-bond substrate materials include low surface energy plastics such as polyethylene, polypropylene, etc., and highly crystalline materials such as polyacetals, polybutylene terephthalate, etc. As a consequence of the difficulty in bonding these materials with cyanoacrylate adhesives, various surface treatments have been employed where such materials require bonding and cyanoacrylate adhesives are otherwise desirably employed.
Examples of such surface treatments include corona discharge exposure of the substrate surface, acid etching, plasma treatment, and the like. These methods are generally unsatisfactory, requiring complex, costly, and substantially non-portable equipment such as corona discharge apparatus or plasma chambers, and/or handling of noxious, toxic, or corrosive fluids in the case of chemical etching of the substrate surface.
In view of the aforementioned deficiencies associated with physical and/or chemical surface roughening techniques, primer compositions have been developed which are applied to the hard-to-bond substrate surface(s), prior to application of the cyanoacrylate bondant medium thereto.
U.S. Pat. No. 4,460,759 to J. Robins, issued July 17, 1984, discloses an adhesive composition comprising an adhesive base which includes an alpha-cyanoacrylate and a stabilizer, in combination with an accelerator including a weakly acidic or weakly basic ionic accelerator compound of the formula MA, wherein M is a cation whose pk.sub.a in aqueous medium is at least about 10, and A is an anion whose pk.sub.a in deprotonation equilibrium reaction is less than or equal to about 0. A has a nucleophilicity constant of less than about 2 when the cation M is an onium cation comprising more than about 8 carbons, the nucleophilicity constant being determined relative to methyl iodide. The adhesive composition has a set time less than one third of that of the adhesive base alone and an overlap shear value of at least 400 lbs per square inch.
Among the cation M species disclosed in the above-mentioned patent are onium cations such as quaternary ammonium cations (see column 4, lines 42-49 of the patent). Illustrative anion A species are exemplified in Table 11 bridging columns 4 and 5 of the patent, as well as the disclosure at column 5, lines 8-15 of the patent.
European Patent Application 129,068 published Dec. 27, 1984 discloses a primer said to be useful for bonding non-polar or highly crystalline substrates together, or for bonding another material, such as a solid material, coating material, paint or printing ink to the substrate, by an alpha-cyanoacrylate. As examples of such difficult-to-bond substrate materials, this publication mentions polyethylene, polypropylene, polybutene, polyfluoroethylene, and their copolymers, as well as other polymeric substrates such as polyethylene terephthalate, polyacetal, nylon and soft (highly plasticized) PVC film. The disclosed primer comprises one or more organometallic compounds, such as metal alcoholates; 1,3-dicarbonyl complexes, carboxylates, hydrocarbon group-containing metal compounds, and heteroatom-containing organometallic compounds. The disclosure at page 2, line 10 to page 3, line 4 of this European Patent Application also describes various prior art primer compositions for enhancing adhesion characteristics of difficult-to-bond substrates. These prior art primer compositions include organic acid solutions of chlorinated polyethylene or chlorinated polypropylene, and fatty acid-modified acrylated alkyd resins.
Japanese Kokai 82/25,378 published Feb. 10, 1982, discloses of solutions of rubber and isocyanates in organic solvents as primers for alpha-cyanoacrylate adhesives.
Kil Pikari, J., et al, Adv. Biomater. 1981, Vol. 4 pp. 101-105, "Adhesion Between Alkyl 2-Cyanoacrylates and Bone Pretreated with 1% Citric Acid," discloses the pretreatment of bone surfaces with 1% citric acid, tartaric acid, or succinic acid, to attempt to improve the adhesion of poly(alkyl 2-cyanoacrylate) thereto. These pretreatments are said not to improve the adhesion strength of the cyanoacrylate bone bond.
West German Offenlegungsschrift 2,950,523 published June 19, 1980, discloses a method of binding urethane formulations to rubber, by coating the rubber surface with a liquid cyanoacrylate adhesive, covering the wet adhesive layer with a polyurethane formulation, and hardening the polyurethane.
Japanese Kokai 74/12,094 published Mar. 22, 1974, describes the treatment of aluminum, polyester, and other substrates, with diethylamine, o-phenylenediamine, or a similar compound, to improve adhesion of such surface to poly(ethyl alpha-cyanoacrylate).
Japanese Kokai 78/35,744 published Apr. 3, 1978, describes the use of polymethyl meth(acrylate) solution in dimethyl ketone as an adhesive composition for porous substrates, e.g., wood, for cyanoacrylate adhesive bonding thereof.
In addition to the above-described primer compositions for enhancing cyanoacrylate adhesive bonding of various substrates, there have been efforts in the art to incorporate in the cyanoacrylate bonding medium additives which function as adhesion promotors for enhancing the bondability of substrates with cyanoacrylate adhesives. Such additives may, however, significantly increase the fixturing time of the cyanoacrylate adhesive composition, thereby adversely affecting the utility of the composition in specific applications, or such additives may limit the utility of cyanoacrylate compositions containing same to bonding of only specific substrate materials, without effecting any improvement in the cyanoacrylate adhesive bondability of other substrate materials.
Accordingly, there is a continuing need in the art for an improved primer composition for achieving enhanced bondability of various substrates with cyanoacrylate adhesives, particularly low surface energy materials or otherwise difficult-to-bond surfaces.
It is therefore an object of the present invention to provide a primer composition which facilitates the cyanoacrylate adhesive bonding of substrate materials such as polyolefins and rubber-modified polyolefins, and which may be useful in bonding a wide variety of low surface energy materials or otherwise difficult-to-bond surfaces.
It is another object of the invention to provide a method of bonding substrates, including surfaces characterized by low surface energy values or otherwise of difficult-to-bond character, by applying to such surfaces a primer composition of a type enhancing the cyanoacrylate adhesive bondability of such surfaces.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.